Effects of shading, fertilization and snail grazing on the productivity of the water fern Azolla filiculoides for tropical freshwater aquaculture.
Water ferns (Azolla spp.) are among the main important floating macrophytes used for feeding farmed animals such as fish, because they have high growth potential and high protein content. Nevertheless, their use as feed requires sustainable mass production, which can be difficult to maintain in field conditions. We performed a first experiment to assess the effects of shading and fertilization levels on the growth of Azolla filiculoides with complementary information regarding the morphology and the chemical composition of the plant cultivated under the different experimental conditions. Plants were cultivated in floating 50-L plastic drums at three fertilization levels ("no", "low" and "high") using an inorganic multi-nutrient fertilizer rich in P (NPK=1:2:1) and maintained under full natural light or shaded by one of two different types of shading materials, transparent polyethylene sheet and 60% shade net, respectively. A second experiment was carried out to evaluate the effects of grazing by the invasive golden apple snail Pomacea canaliculata on A. filiculoides previously cultivated without addition of fertilizer (treatment "no") or in high fertilizer concentrations (treatment "high"). Fertilization levels and shading materials significantly affected the growth of Azolla. The highest productivity was reached using the highest fertilization level under direct sunlight. Azolla produced in these culture conditions was preferentially grazed by snails compared to Azolla cultivated without added fertilizer. Based on these findings, we make recommendations regarding the best culture conditions for A. filiculoides in ponds for its use as sustainable fish feed.